Ultra-high speed, active polymer-silica hybrid, single control voltage MMI-based 1-by-N packet switch and WG-based WDM packet router/TDM converter and methods of making same

ABSTRACT

A structure for effecting a transition from a passive waveguide to an active waveguide or from an active waveguide to a passive waveguide of the present invention. The inventive device comprises a first cladding; a first core disposed within the first cladding; and a ground plane disposed over the first cladding and the core. A second cladding is disposed on the ground plane. A second core is disposed on the second cladding. A third cladding is disposed on the second cladding and the second core and an electrode is disposed on top of the third cladding. The inventive structure enables the construction of a novel an advantageous switch comprising an input port; an output port; and plural waveguides disposed between the input port and the output port. Each waveguide includes a first cladding; a first core disposed within the first cladding; and a ground plane disposed over the first cladding and the core. A second cladding is disposed on the ground plane. A second core is disposed on the second cladding. A third cladding is disposed on the second cladding and the second core and an electrode is disposed on top of the third cladding. The inventive structure also enables a unique and advantageous router design comprising an active tuned arrayed waveguide grating and switching logic for controlling the grating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fiber optic devices used forcommunication and other applications. More specifically, the presentinvention relates to planar lightwave circuits.

2. Description of the Related Art

As is well-known in the art, many optical circuits use optical guides:planar lightwave circuits constructed with patterned silicon-dioxidelayers on a silicon substrate.

Planar lightwave circuits (PLCs) made of low-loss silica promise to makesignificant impact as they reach commercial viability. Indeed,silica-based planar lightwave circuits—passive optical waveguidestructures made using photolithographic techniques—comprise one of themost dynamic segments of the photonics field. Among their key virtuesare extremely low propagation loss (0.01 dB/cm), excellent fibercoupling loss (0.1 dB for low index contrast waveguides), ease ofdefining complex structures such as Arrayed Waveguide Grating (AWGs) andMach-Zehnder arrays using photolithographic fabrication processes, modecompatibility with optical fibers, and physical robustness. However,silica is a passive material with no electrically controlled phaseshifting ability except for slow thermo-optic index modulationtechniques.

In general, passive materials, previously used for routing andswitching, offer low losses but suffer from low speeds and are notelectrically responsive. Active materials, used for modulators and otherdevices, offer higher speeds but suffer from higher losses as well.

Consequently, a need exists in the art for a system and method forintegrating low loss passive materials with active high speedelectro-optic materials to make more sophisticated devices such asmodulators, routers, and switches using fabrication processes compatiblewith both passive and active materials.

SUMMARY OF THE INVENTION

The need the art is addressed by the device for effecting a transitionfrom a passive waveguide to an active waveguide or from an activewaveguide to a passive waveguide of the present invention. The inventivedevice comprises a first cladding; a first core disposed within thefirst cladding; and a ground plane disposed over the first cladding andthe core. A second cladding is disposed on the ground plane. A secondcore is disposed on the second cladding. A third cladding is disposed onthe second cladding and the second core and an electrode is disposed ontop of the third cladding.

The inventive device enables the construction of a novel andadvantageous switch comprising an input port; an output port; and pluralwaveguides disposed between the input port and the output port. Eachwaveguide includes a first cladding; a first core disposed within thefirst cladding; and a ground plane disposed over the first cladding andthe core. A second cladding is disposed on the ground plane. A secondcore is disposed on the second cladding. A third cladding is disposed onthe second cladding and the second core and an electrode is disposed ontop of the third cladding.

The inventive device also enables a unique and advantageous routerdesign comprising an active tuned arrayed waveguide grating andswitching logic, for controlling the grating. In the illustrativeembodiment, the grating includes an input port; an output port; andplural waveguides disposed between the input port and the output port.Each waveguide includes a first cladding; a first core disposed withinthe first cladding; and a ground plane disposed over the first claddingand the core. A second cladding is disposed on the ground plane. Asecond core is disposed on the second cladding. A third cladding isdisposed on the second cladding and the second core and an electrode isdisposed on top of the third cladding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a-b are perspective views of the first two sections (“taper” and“riser” respectively) comprising the adiabatic transition structureconstructed in accordance with the teachings of the present invention.

FIG. 1c shows the active section of the transition, which immediatelyfollows the riser. The active section follows the transition structure,and is where electro-optic modulation takes place.

FIG. 2 superimposes electromagnetic simulation results onto thetaper/riser transition, showing that the optical field is properlyrouted from the silica waveguide below to the active section, in properposition with respect to the modulation electrodes.

FIGS. 3a-c are diagrams which illustrate a 3-D structure and transitionfabrication process implemented in accordance with the teachings of thepresent invention.

FIG. 4 is a schematic diagram showing an illustrative implementation ofa switch architecture with N=8 in accordance with the teachings of thepresent invention.

FIG. 5 is a block diagram showing an illustrative implementation of anultra-high speed wavelength division multiplexed (WDM) packetrouter/time division multiplexed (TDM) converter in accordance with theteachings of the present invention.

DESCRIPTION OF THE INVENTION

Illustrative embodiments and exemplary applications will now bedescribed with reference to the accompanying drawings to disclose theadvantageous teachings of the present invention.

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those havingordinary skill in the art and access to the teachings provided hereinwill recognize additional modifications, applications, and embodimentswithin the scope thereof and additional fields in which the presentinvention would be of significant utility.

The present invention addresses the above-identified need in the art byproviding a hybrid solution, with low loss silica routing signals to andfrom active sections which can provide wideband index modulation. Twoleading conventional modulation mechanisms for telecommunicationwavelengths, LiNbO3 and InP, are both poorly suited for hybridintegration with silica PLCs. LiNbO3 waveguides are formed byion-diffusion, an intrinsically incompatible process. Greater successhas been achieved with silica/InP integration. However, since the activesections suffer excessive index mismatch from silica, the strategieswhich have been pursued involve adiabatic spot size converters aidingbutt-coupling. This requires simultaneous fabrication of both silica andInP sections, as well as simultaneous horizontal and vertical alignment,both of which are challenging.

On the other hand, an electro-optic polymer developed by Pacific WaveIndustries holds considerable promise for successful integration. Inaddition to possessing properties which make it an excellentelectro-optic material—high nonlinear effect, good optical/microwavevelocity match, intrinsically fast response—processing steps for thepolymer are conventionally photolithographic, and the refractive indexis in a fortuitous range. The index difference between silica and theactive polymer (1.45 vs. 1.6) is large enough that straightforward buttcoupling is unlikely to work well. However, the index difference is suchthat the adiabatic coupling approach utilized in the present inventionis effective.

The present teachings illustrate that a composite structure, where thehigher index polymer lies next to the silica waveguide core, willexhibit a fundamental mode where almost all of the energy is confined inthe polymer region. If a transition to such a region from a no-polymerregion is made sufficiently slowly (“adiabatically”) then energy shouldremain in the fundamental mode as the mode itself evolves. This is thephysical basis for the operation of the claimed invention.

An adiabatic transition has another intrinsic advantage: the transitioncan be vertical. This implies that processing can be performed indisparate steps layer by layer, requiring almost no modification toexisting silica waveguide fabrication steps. The invention then involvesa fabrication of a complete silica planar lightwave circuit usingconventional methods, then coating and etching polymer segments andelectrodes on top where needed. In each step, alignment of only thehorizontal dimension is required.

Thus, the present invention provides a high-performance silica toelectro-optic polymer transition. The illustrative embodiment relates toMach-Zehnder modulators, however the teachings provided herein may beutilized to fabricate many other devices, of increasing complexity,without departing from the scope of the present teachings.

The structure of an adiabatic transition constructed in accordance withthe teachings of the present invention is shown in FIGS. 1a—c. While thetransition is a single continuous device, it is illustrative to considerit as being composed of three sections, here dubbed “taper,” “riser,”and “active section.”

FIG. 1a shows the taper section, whose purpose is to gradually(adiabatically) “suck” the optical field being guided by the passivesilica waveguide into the polymer deposited above. It is composed of(PWI name/patent reference here) polymer, photolithographically shapedto be a vertical exponential. Because the structure imposes a change ineffective index from 1.45 to near 1.6, a long length is required. In theillustrative embodiment shown here, the taper is 2 mm long. In addition,the active polymer is in the form of a rib waveguide. Nonetheless, thoseof ordinary skill in the art will appreciate that other shapes andlengths may be utilized without departing from the scope of the presentteachings.

FIG. 1b shows the riser section, whose purpose is to vertically displacethe optical field, which by now is being guided by the polymer, so as tomake room for the electrodes in the active section to follow. It iscomposed of the active core polymer sitting atop a passive lowercladding, composed of a suitable material such as UV-15. Because theriser merely turns the optical field without a large change in waveguideeffective index, it is much shorter than the taper. In the illustrativeembodiment shown here, the riser is 200 microns long. Both the bottomcladding and the active polymer layers of the riser arephotolithographically defined to be a vertical S-curve.

FIG. 1c shows the active section, which differs from the riser sectiononly in that electrodes are introduced. The ground plane—occupying thelower half of the bottom cladding section in the riser—and the topelectrode together form a conventional microstrip. In the illustrativeembodiment, the ground plane is a layer of gold or other suitableconductor of adequate thickness, i.e. 2 microns, and the top electrodeis a layer of gold or other suitable conductor and is separated from theground plane by 7.5 microns. These dimensions achieve a desirablemicrostrip impedance of 50 ohms, but those of ordinary skill in the artwill appreciate that other dimensions may be utilized without departingfrom the scope of the present teachings.

It should be noted that, in the illustrative embodiment and anyequivalent design, the modal confinement properties of the activepolymer are such that almost no optical field intercepts the electrodesin the active section, allaying any concerns regarding optical loss dueto conductive interaction with the electrodes. This is seen clearly inFIG. 2, which shows a scale drawing of the taper/riser structure, with avectoral electromagnetic simulation of the optical field superimposed.At the input to the active section, which follows the riser, the opticalfield is properly positioned with respect to the electrodes, and highlyconfined in the active polymer.

FIGS. 3a—c are diagrams which illustrate a 3-D structure and transitionfabrication process implemented in accordance with the teachings of thepresent invention. For simplicity of the drawing, the transitionsections are shown as linear height tapers rather than the exponentialand S-curve structures shown above. These basic steps are required:deposition of the gold ground plane, coating/etching of the risersection, coating/etching of the active polymer taper, coating of theupper polymer cladding, and finally deposition of the upper electrode.Alignment between the silica waveguide wafer and the, polymer patterningmasks is maintained with cooperative marks designed into the former.

Hence, as illustrated in FIG. 3a, the structure 3 is fabricated bydepositing a conductive (e.g., gold) ground plane 32 on a silicawaveguide 34. Note also that conductive alignment marks 36 may also bedeposited on the silica waveguide 34.

Next, the lower cladding layer for the riser 38 is spun on over theground plane 32 and silica waveguide 34. The lower cladding layer 38 maybe UV-15 or other suitable optical material. Photoresist is then spunon, then grayscale-etched to transfer the S-curve pattern to the riser'slower cladding.

Next the same process is repeated for the active layer. A layer of the(active PWI polymer) is spun on, followed by photoresist, followed bygrayscale-etch to transfer both the taper section's exponential and theriser section's S-curve profile to the active polymer.

In the illustrative embodiment, grayscale masks are used for the etchingof the 3-D transition structures. High quality grayscale masks with therequired resolution are now commercially available, making their usepreferable to other methods such as shadow-masking.

ULTRA HIGH SPEED SINGLE CONTROL VOLTAGE 1×N INTEGRATED OPTICAL SWITCH

The inventive transition may be utilized in a variety of applications,e.g., high speed switching in photonic communications networks. AMulti-Mode Interferometer (“MMI”) based switch architecture offers manyadvantages such as a generalized 1×N and N×N structure in a singledevice and good tolerance to fabrication errors. However, to push theswitching speed up to the needed gigahertz (GHz) range, the N separateelectrical signals required become increasingly cost-prohibitive. Bytaking advantage of certain symmetries in the well-known phaserelationships in MMI's and utilizing the present teachings, a device maybe constructed capable of switching an optical signal into N outputports using a single electrical control signal. (In addition, the uniqueability to reverse the poling-induced electro-optic coefficient inactive polymers manufactured and sold by Pacific Wave Industries mayyield in a reduction by a factor of 2 in either the interaction lengthor the switching voltage required.)

FIG. 4 is a schematic diagram showing an illustrative implementation ofa switch architecture with N=8 in accordance with the teachings of thepresent invention. As illustrated in FIG. 4, the switch 50 includes a1×N MMI 52, an N×N MMI 54 and eight waveguides 61-68 disposedtherebetween. Those skilled in the art will appreciate that theinvention is not limited to 8 switched outputs. The teachings of thepresent invention may be extended to a number of waveguides required fora given application without departing from the scope thereof.

The MMI's and all other optical fan-out waveguides required to connectthe claimed device to external optical fibers are constructed usingsilica-PLC technology, and each of the eight waveguides is fabricated asa hybrid structure 10 discussed above. The active sections of the hybridstructure perform the switching action. The electrodes of the eightwaveguides 61-68 are driven by a source 56 such that the electricalwaves are aligned in phase.

In accordance with the present teachings, the source 56 applies avoltage V such that an input is directed to an output port in accordancewith a predetermined sequence. The voltage V is chosen such that theshortest electrode provides a desired phase shift θ, where θ=360/N=45degrees where, as here, N=8. That is, applying nV, where n is 1 . . . N,will result in an input being directed to an output port 1-8. The outputsequence is a function of N; in the case of N=8, the sequence is{5,3,1,2,4,6,8,7}. Those skilled in the art will recognize that therandom-looking output sequence is in fact a consequence of thephase-relationships between input/outputs of MMI's. In addition, theorder in which the sequence is traversed (in this example,5-3-1-2-4-6-8-7 or 7-8-6-4-2-1-3-5) depends on the polarity of theapplied switching field (that is, “+ on ground plane” or “− on groundplane”).

The physical basis for the switching action in the claimed invention isthe proper control of the relative phase shift which is imparted in theactive section of each waveguide. The novelty of the invention lies in aparallel method of achieving these phase shifts, using special; relativelengths of the active sections in each waveguide. As mentioned above,this implementation is particularly well suited for high-speedoperation. The prescription for the correct active section lengths isbest described by the following “rules.” These rules are stated in termsof the applied angular phase shift, which is the physically fundamentalproperty. The translation from angular phase shift to physical length isan engineering design parameter, as discussed below.

1) Waveguide #1 never need not have an electrode on it.

2) Define Theta0=360/N in degrees.

3) If N is odd, there are a total of(N−1) electrodes with relativelengths so as to implement the phase shifts:

−Theta0

+Theta0

−2Theta0

+2Theta0

. . .

−((N−1)/2)Theta0

+((N−1)/2)Theta0

4) If N is even, there are a total of (N−1) electrodes with relativelengths so as to implement the phase shifts:

−Theta0

+Theta0

−2Theta0

+2Theta0

. . .

−((N−2)/2)Theta0

+((N−2)/2)Theta0

180 deg

In a device whose active lengths are designed following these rules, theswitching voltage logic is: application of a voltage, corresponding ton*Theta0 of phase shift on the shortest electrode, n=1. . . N, willswitch the optical signal through the output ports in the specialsequence which is a function of N, as described above.

Note that the appearance of +/−pairs permits the use of oppositely poledactive polymer segments to achieve the required phase shifts with onelength. This cuts the device length down by nearly a factor of 2 (e.g.can generate −30 rather than +330 degrees).

As mentioned above, each of the waveguides 61-68 has an active polymersection whose length is chosen as a compromise between minimizing loss(short active section) and minimizing switching voltage (long activesection). Consider an illustrative embodiment where the longest activesection length is 2 cm. The current state of the art in PWI's activepolymer is such that a 2 cm active length results in a V-pi of 2v (thoseskilled in the art will recognize V-pi as the voltage necessary toinduce a 180 degree optical phase shift.) Therefore such a device willrequire a maximum of about 16 volts to access all output ports for N=8.

ULTRA HIGH SPEED WDM PACKET ROUTER/TDM CONVERTER

The Arrayed Waveguide Grating (AWG) has been extensively studied as anintegrated optics device for demultiplexing WDM (Wavelength-DivisionMultiplexed) signals. The most common configuration is a static one,where an array of waveguides with specific lengths between twostar-couplers, perform the equivalent function of a prism or grating,splitting multiple wavelengths in a single input waveguide into separateoutput waveguides. This permits AWGs to function as wavelength routersand Add-Drop Modules (ADM), crucial functions for WDM networks.

Active tuning of AWG's can be achieved by phase-shifting the waveguidesin the array. Because coherence of the beams in the array must bemaintained for proper localization of the fields at the outputwaveguides, each waveguide in the array should be phase-shifted by thesame amount. The effect of this tuning is to shift the wavelength of thefield localized at each output. AWG's tuned in this manner have beenreported in the literature, mostly using the thermo-optic effect onsilica or passive polymer devices.

The inventive device 10 achieves this tuning by depositing an adiabatictransition to and microstrip electrode atop each waveguide in the AWG.The electrodes are driven by a single microwave source. While the tuningmechanism resembles that of thermo-optically tuned AWG's, the fact thatit is orders of magnitude faster permits interesting applications.Specifically, a fast tunable AWG can be used to route packets carried inmultiple WDM channels to a single channel. If those packets comprise asingle meaningful data stream, then the tuning speed corresponds to thepacket transmission rate. If the packets comprise multiple meaningfuldata streams, then the device functions as a WDM-TDM converter, and thetuning speed translates directly to the time-slice on the output TDMchannel. The basic requirement, fulfilled by the use of fast EO polymersin the inventive device, is that the device keep up with the packet ratein the input WDM channel.

FIG. 5 is a block diagram showing an illustrative implementation of anultra-high speed wavelength division multiplexed (WDM) packetrouter/time division multiplexed (TDM) converter in accordance with theteachings of the present invention.

In accordance with present teachings, the hybrid AWG 70 is fed withpacketized wavelength-multiplexed data and outputs a single opticalchannel with temporally-interleaved packets under control of switchinglogic 92. It is in this sense that the device converts from WDM to TDM.

The switching logic 92 generates a routing signal which is synchronizedto the same data clock as the packets in the WDM stream. The routingsignal tunes the AWG 70 to localize different wavelengths (channels) atthe single output waveguide, in sync with the data clock. Essentially,it is a “fast steering prism.” The switching logic (software) determinesthe actual packet sequence which is output; as such, it isapplication-dependent and plays no substantive role in the operation ofthe claimed invention.

Note that the optical signal in each output waveguide is composed ofpackets with different wavelengths. No frequency conversion is performedby the device—its basic function is to impose a set temporal order,synchronized with the external clock, to the wavelength variations ineach output waveguide. Since the L and C-Band WDM channels fitcomfortably within the response wavelength envelope of typicalhigh-speed indium gallium-arsenide (InGaAs) photodetectors, the presenceof multiple wavelengths in the output is not necessarily of concern. Aslong as chromatic dispersion is properly controlled (say with the use ofDSF-fiber) or of no consequence (say in a short to medium lengthmetro-network), the photodetector-receiver views the signal as a singlestream of packets. In short, the system 90 imposes a temporal orderwhich makes the packet stream time-division-multiplexed.

Finally, note that multiple devices may be used in parallel to generatemultiple independent streams, limited only by the signal to noise ratiothereof.

Thus, the present invention has been described herein with reference toa particular embodiment for a particular application. Those havingordinary skill in the art and access to the present teachings willrecognize additional modifications, applications and embodiments withinthe scope thereof.

It is therefore intended by the appended claims to cover any and allsuch applications, modifications and embodiments within the scope of thepresent invention.

Accordingly,

What is claimed is:
 1. A device for effecting a transition from apassive waveguide to an active waveguide or from an active waveguide toa passive waveguide comprising: a first cladding; a first core disposedwithin the first cladding; a ground plane disposed over the firstcladding and the core; a second cladding disposed on the ground plane; asecond core disposed on the second cladding; a third cladding disposedon the second cladding and the second core; and an electrode disposed ontop of the third cladding.
 2. The invention of claim 1 wherein the firstcladding is fabricated with a passive material.
 3. The invention ofclaim 2 wherein the first cladding is fabricated with silicon dioxide.4. The invention of claim 3 wherein the first cladding has an index ofrefraction of 1.4568.
 5. The invention of claim 4 wherein the first coreis fabricated with silicon dioxide.
 6. The invention of claim 5 whereinthe first core has an index of refraction of 1.4612.
 7. The invention ofclaim 1 wherein the second cladding is a polymer cladding.
 8. Theinvention of claim 7 wherein the second cladding has an index ofrefraction of approximately 1.45.
 9. The invention of claim 1 whereinthe second core is an active core.
 10. The invention of claim 9 whereinthe active core has an index of refraction of approximately 1.6.
 11. Theinvention of claim 1 wherein the third cladding is a polymer cladding.12. The invention of claim 11 wherein the third cladding has an index ofrefraction of approximately 1.45.
 13. The invention of claim 1 whereinthe ground plane is gold.
 14. The invention of claim 1 wherein theelectrode is gold.